Insulated gate bipolar transistor

Abstract: A power device integrated structure comprises a semiconductor substrate (5) of a first conductivity type, a semiconductor layer (3,4) of a second conductivity type superimposed over said substrate (5), a plurality of first doped regions (2) of the first conductivity type formed in the semiconductor layer (3,4), and a respective plurality of second doped regions (11) of the second conductivity type formed inside the first doped regions (2); the power device comprises: a power MOSFET (M) having a first electrode region represented by the second doped regions (11) and a second electrode region represented by the semiconductor layer (3,4); a first bipolar junction transistor (T2) having an emitter, a base and a collector respectively represented by the substrate (5), the semiconductor layer (3,4) and the first doped regions (2); and a second bipolar junction transistor (T1) having an emitter, a base and a collector respectively represented by the second doped regions (11), the first doped regions (2) and the semiconductor layer (3,4); the doping profiles of the semiconductor substrate (5), the semiconductor layer (3,4), the first doped regions (2) and the second doped regions (11) are such that the first and second bipolar junction transistors (T2,T1) have respective first and second common base current gains sufficiently high to cause said bipolar junction transistors to be biased in the high injection region, so that carriers are injected from the substrate (5) into the semiconductor layer (3,4) and from the second doped regions (11), through the first doped regions (2), into the semiconductor layer (3,4), the conductivity of the semiconductor layer (3,4) is thus modulated not only by the injection of minority carriers from the substrate (5), but also by majority carriers injected from the doped regions (11) into the first doped regions (2) and collected by the semiconductor layer (3,4). The first and second common base current gains summed are less than unity to prevent a parasitic thyristor from triggering on. The power device functions as an IGBT, having a reduced on-state voltage.

Description: The present invention relates to an insulated gate bipolar transistor with low saturation voltage. It is known that the higher the drain-source voltage a power MOSFET must withstand, the thicker the epitaxial drain layer wherein the power MOSFET elementary cells are formed. This Layer is lightly doped, and has a resistivity of the order of 50 Ohm/cm; since for maximum voltage ratings of 1000-2000 V a 100 µm-thick layer must be provided, a significant resistance is introduced in series to the conductive channel, which contributes to increasing the power MOSFET on-state resistance Ron. The power MOSFET Ron is thus directly related to the power MOSFET breakdown voltage BV, so that the higher BV the higher Ron. This is disadvantageous since it causes significant voltage drops across the power device, and large power dissipations. The Insulated Gate Bipolar Transistor (IGBT) has been introduced to overcome the limitations of the power MOSFET structure in the high-voltage (1000-2000 V) application field. An IGBT is known from eg. DE-A-4 208 695. The fundamental idea that led to the IGBT is the introduction of conductivity modulation of the drain layer: to this purpose, the substrate of the chip is doped of a conductivity type opposite to that of the drain layer, so to obtain a PN junction; when the IGBT is turned on by applying a proper bias voltage to the gate electrode, carriers flow from the emitter electrode through the conductive channel, and drift through the drain Layer (which for this reason is also called "drift layer"); the substrate-drift layer junction becomes forward biased, and carriers of the opposite sign are injected from the substrate into the drift layer, whose conductivity is thus enhanced. The problem is that the IGBT structure has inherently associated a parasitic three-junction device (i.e. a thyristor); considering for example an N-channel device, the parasitic thyristor is made up of an NPN transistor with emitter and base respectively represented by the source and body regions of the elementary cells, and collector represented by the drift layer, and a PNP transistor with collector, base and emitter respectively represented by the body regions of the elementary cells, the drift layer and the substrate. If the parasitic thyristor triggers on, a low-resistance path establishes between the source regions and the substrate, the current flowing through the device theoretically diverges, and the IGBT is destroyed. It is therefore extremely important to prevent the parasitic thyristor from triggering on. To this purpose, the designers' efforts conventionally aim at preventing the parasitic NPN from entering conduction. One way to achieve such a result is by reducing as far as possible its common base current gain αn; the dopant concentration in the source regions and in the body regions of the elementary cells, which respectively form the emitter and the base of the parasitic NPN, is thus properly chosen so that the difference in said concentration does not exceed a given value. Another way to assure that the parasitic NPN does not enter conduction is to clamp its base-emitter voltage to zero: the body regions are thus short-circuited by the emitter metal layer to the source regions; also, to reduce the base-emitter resistance Rbe of such transistor, i.e. the physical resistance along the body region of the elementary cells, the body regions are provided with a heavily doped portion, and the Length of the source regions is controlled so that they are almost completely internal to the heavily doped portion of the body regions. The equivalent electrical circuit of the structure thus obtained comprises a power MOSFET and a PNP transistor (the MOSFET's drain and source being connected to the PNP's base and collector, respectively), since the NPN transistor is substantially eliminated. In this way, however, conductivity modulation of the drift layer can only take place from the substrate; since the average lifetime, and consequently the diffusion length, of holes in the drift layer is low (to increase the IGBT speed performances the lifetime of holes in the drift layer is deliberately lowered by the introduction of lifetime killers, and a heavily doped "buffer" layer is provided at the bottom of the drift layer to facilitate the collection of holes when the IGBT is switched off), the conductivity modulation effect is quite limited, and the output resistance is high. In these conditions, it can be shown that the collector-emitter voltage in saturation (VCEsat) is proportional to the square of the drift layer thickness, and is therefore quite high. This causes static losses in the device. In view of the state of the art described, it is an object of the present invention to provide an IGBT in which the collector-emitter saturation voltage is lower than in conventional IGBT structures, so that static losses can be reduced. According to the present invention, such object is attained by means of an insulated gate bipolar transistor as set forth in claim 1. Specific embodiments are disclosed in the dependent claims. Thanks to the present invention, when the IGBT is turned on transistor action takes place both in the PNP and in the NPN parasitic transistors, even if the thyristor latching is prevented; the conductivity of the power MOSFET drain layer is thus modulated not only by the injection of (minority) carriers from the substrate, but also by (majority) carriers injected from the source regions into the body regions of the elementary cells, and collected by the drain layer. In this way, the conductivity modulation of the drain Layer is much more effective, and the collector-emitter voltage in saturation (VCEsat) is lower, since it becomes proportional to 1/2 the square of the drift layer thickness. The equivalent electrical circuit of the IGBT according to the present invention comprises a power MOSFET, and a PNP and an NPN transistors with base and collector cross-connected to each other in a thyristor-Like configuration; both transistors become forward-biased when the IGBT is driven on, but the thyristor latching is prevented since αn + αp < 1. The features of the present invention will be made more evident by the following detailed description of a particular embodiment, illustrated as a non-limiting example in the annexed drawings, wherein: Figure 1 is a cross-sectional view of an IGBT according to the present invention; Figure 2 is a diagram showing the doping profiles of the various semiconductor regions of the IGBT shown in Figure 1; Figure 3 is an electric equivalent circuit of the IGBT according to the present invention; Figure 4 shows the flows of carriers when the IGBT is on; Figure 5 is a diagram showing the profiles of the concentration of the injected carriers when the IGBT is on. An IGBT according to the present invention, for example of the N channel type, is made up of a plurality of elementary functional units 1 comprising a P type body region 2, typically with polygonal plan, formed in a lightly doped N type semiconductor layer 3 called "drift layer"; the N type layer 3 is disposed over a heavily doped N type semiconductor layer 4 called "buffer layer", which is in turn disposed on a heavily doped P type substrate 5. Each body region 2 comprises a central heavily doped portion 6 called "deep body region", and a lateral more lightly doped portion 7 called "channel region". Inside each body region 2 a heavily doped source N type region 11 is formed. The surface of the N type layer 3 is covered by a conductive insulated gate layer 8 comprising a thin oxide layer 9 called "gate oxide" and a conductive material layer 10 (generally of polysilicon) called "gate layer"; the insulated gate layer 8 layout forms a mesh structure. The gate layer 10 is connected to a gate electrode G of the IGBT. An insulating material layer 12 covers the insulated gate layer 8; contact windows 13 are opened in the insulating material Layer 12 over the elementary units 1 to allow an overlying metal layer 14 to contact the source regions 11 and the deep body regions 6; the metal layer 14 forms an emitter electrode E of the IGBT. A metal layer 15 is also provided at the bottom surface of the substrate 5 to form a collector electrode C of the IGBT. From the electrical point of view, the IGBT comprises: a power MOSFET M formed by the source regions 11, the channel regions 7 and the N type layer 3; an NPN bipolar junction transistor T1 formed by the source regions 11, the body regions 2 and the N type layer 3; and a PNP bipolar junction transistor T2 formed by the substrate 5, the N type layer 3 and the body regions 2. The two transistors T1 and T2 are cross-connected to form a three-junction device, i.e. a thyristor (Fig. 3). The doping profiles Nd and Na of the various semiconductor regions, shown in Figure 2, are such that both the NPN transistor T1 and the PNP transistor T2 have respective common base current gains αn and αp sufficiently high to cause transistor action to take place, the sum αn + αp being however kept below unity to prevent the thyristor from triggering on; preferably, said doping profiles are such that αn and αp are both a little lower than 0.5. With respect to the known IGBT structures, wherein the doping profiles are such that the NPN transistor α n is almost zero, the source regions 11 are more heavily doped, to increase the emitter efficiency of T1: while in fact in conventional IGBT structures the peak dopant concentration of the source regions 11 is of the order of 10 19 atoms/cm 3 , in the IGBT according to the present invention the source regions 11 have a peak dopant concentration at least one order of magnitude higher (10 20 atoms/cm 3 ); additionally, the deep body regions 6 are more lightly doped (in conventional IGBT structures, the doping profile of the deep body regions 6 is such that under the source regions 11 the dopant concentration is of the order of 10 18 - 10 19 atoms/cm 3 ). In conventional IGBT structures the buffer layer 4 is provided to facilitate the collection of minority carriers present in the N type layer 3 when the IGBT is switched off; in fact, the buffer layer 4 limits the diffusion length of holes which modulate the conductivity of the the N type layer 3 (i.e. the PNP transistor αp is reduced). Consequently, in conventional IGBT structures there is a trade-off between the IGBT switching performances and its collector-emitter voltage in saturation: the peak dopant concentration of the buffer layer 4 is of the order of 10 16 atoms/cm 3 . In the IGBT according to the present invention, since the conductivity of the N type layer 3 is not only modulated by holes injected into the it from the substrate 5, but also by electrons injected into the collector of T1, such a trade-off does not exist; on the contrary, the doping level of the buffer layer 4 should be sufficiently high to reduce the PNP transistor emitter efficiency. The peak dopant concentration of the buffer layer 4 is thus increased to at least 10 17 - 10 18 atoms/cm 3 , so that αp achieves the desired value of approximately 0.5. Thus, the IGBT according to the present invention not only has a lower collector-emitter saturation voltage, but is also intrinsically faster than conventional IGBTs. When a proper bias voltage is applied to the gate electrode G, an inversion channel is formed in the channel regions 7 of the elementary functional units 1, and the power device turns on. Electrons enters the power device from the emitter electrode E, flows along the inversion channel, and drift in the N type layer 3 to the substrate 5, where they are collected at the collector electrode C. The buffer layer 4-substrate 5 junction becomes forward biased, and T2 is turned on; the base current of T2 turns T1 on, so that electrons from the source regions 11 are vertically injected into the body regions 2 and are collected by the N type layer 3. This last layer is therefore modulated in conductivity both from the bottom (by holes injected from the substrate 5) and from the top (by electrons flowing from the emitter to the collector of T1) (Fig. 4). The concentration profiles of the conductivity-modulating electrons and holes in the N type layer are shown in Figure 5 (e and h, respectively).

Claims: Insulated gate bipolar transistor comprising a semiconductor substrate (5) of a first conductivity type, a semiconductor layer (3,4) of a second, opposite conductivity type disposed over said substrate (5), a plurality of first doped regions (2) of the first conductivity type formed in said semiconductor layer (3,4) and having a central, heavily doped portion (6) and a lateral, more lightly doped portion (7), and a respective plurality of second doped regions (11) of the second conductivity type formed inside said first doped regions (2), wherein the doping profiles of the semiconductor substrate (5), the semiconductor layer (3,4), the first doped regions (2) and the second doped regions (11) are such that a first bipolar junction transistor (T2), having emitter, base, collector respectively formed by the substrate, the semiconductor layer, the first doped regions, and a second bipolar junction transistor (T1), having emitter, base, collector respectively formed by the second doped regions, the first doped regions, the semiconductor layer, have respectively a first and a second common base current gains (α p ,α n ) which are: a) individually sufficiently high to cause both the first and second bipolar junction transistor to be forward biased when the insulated gate bipolar transistor is on, so that carriers are injected into the semiconductor layer both from the substrate and from the second doped regions through the first doped regions to modulate the conductivity of the semiconductor layer, and b) such that their sum is less than unity to prevent a parasitic thyristor formed by the first and second bipolar junction transistors from triggering on, wherein the second doped regions (11) have a dopant concentration of the order of 10 20 atoms/cm 3 , and the difference in dopant concentration between the second doped regions (11) and the heavily doped portions (6) of the first doped regions (2) is about two orders of magnitude, and wherein the semiconductor layer comprises a lightly doped portion (3) and a heavily doped portion (4) directly on the substrate (5), the substrate (5) having a dopant concentration of the order of 10 20 atoms/cm 3 , and the heavily doped portion (4) of the semiconductor layer (3,4) having a dopant concentration of the order of 10 17 atoms/cm 3 or more. Transistor according to claim 1, characterized in that the first conductivity type is the P type, and the second conductivity type is the N type. Transistor according to claim 1, characterized in that the first conductivity type is the N type, and the second conductivity type is the P type.

Inventors: FERLA GIUSEPPE, FRISINA FERRUCCIO, FERLA, GIUSEPPE, FRISINA, FERRUCCIO
Applicants: CONS RIC MICROELETTRONICA, ST MICROELECTRONICS SRL, CONSORZIO PER LA RICERCA SULLA<BR>MICROELETTRONICA NEL MEZZOGIORNO, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H01L29
IPC codes: H01L29